1. Field of the Invention
The present invention relates to a gloss applicator and an image forming apparatus, and more specifically relates to printed output gloss control.
2. Description of the Related Art
As is known, in an image forming apparatus employing an electrophotographic method, an electrostatic latent image formed on a photoreceptor or a latent image carrier is rendered visible by toner, after which the visible toner image is transferred onto a recording medium such as a recording sheet and fixed thereon, thereby obtaining printed output.
Images obtained as printed output include not only monochrome images but also multi-color images such as full-color images. Concerning the appearance and color-reproducibility of the image, glossiness of the image and that of the recording medium bearing the image thereon are very important. In particular, matching of the glossiness of the two is necessary to make the image appear natural, as disclosed in JP-2004-32593-A.
As a factor defining the glossiness, there is melting/permeating property of the toner with respect to the recording medium. As a device exploiting the melting/permeating property, a fixing device is used.
The typical fixing device may be configured to use a heat-roller fixing method, including a fixing roller with a built-in heat source such as a halogen lamp and a pressure roller in contact with the fixing roller in combination. In addition, there is another method called a belt-fixing method using a belt having a low thermal capacity.
As a known structure using the belt-fixing method, a fixing belt with an elastic surface formed of silicon rubber and a release layer formed of fluorine resins on the surface of the silicon rubber surface is stretched over a plurality of rollers; a pair of a stretched roller and a pressure roller is opposed to each other with the fixing belt in between; and a heat source such as a halogen lamp is built into the stretched roller.
In such a fixing device, when a recording sheet on which a toner image is formed is passed between the fixing belt and the pressure roller, color toner on the sheet is melted by heat and is fixed onto the recording sheet or medium. Because a fixing nip can arbitrarily be set due to its belt structure in the fixing-belt method, there are advantages such as the ability to fix at a lower temperature (i.e., energy-saving effect) and improved separability of the recording medium from the belt. On the other hand, the recording medium bearing a toner image being a target of fixation thereon varies from a plain paper having a concave-convex surface with less glossiness to a highly glossy sheet such as art paper, coated paper, slightly coated paper, and the like, having a smooth surface and higher glossiness. The use of the glossy paper for fixation has come to be desired recently.
Of these paper materials, the art paper and the coated paper each have a glossier surface than the plain paper because a coating layer formed of resins, for example, is coated on the surface of such paper. Further, in recent years, a low-gloss sheet or matt coated paper as the plain paper has come to be used to make the paper document look elegant. As a result, the image forming apparatus is required to stably output a high-quality image that the user desires even in forming a toner image on such various types of paper.
Conventionally, there is known a structure to obtain a desired glossiness, in which a recording sheet which has passed through the fixing process after image transfer is again heated so that the toner image is softened, and a gloss applicator which can apply smoothness similar to the smooth surface of the sheet conveyance belt to the softened toner image surface and further cool the heated toner image surface to make it easier for the recording sheet to be separated from the sheet conveyance belt. (See, for example, JP-2004-32593-A, described above, and JP-2009-14876-A.)
When improving the image quality by a higher glossiness of the toner image, as disclosed in the above patent-related documents, it is known that the smoothness of the surface of the conveyance member or the belt contacting the toner image when softening the toner image surface exerts a good effect. On the other hand, the recording sheet on which the glossiness-improved toner image improved by assimilating the smooth surface of the conveyance member is carried is cooled so that the toner surface is solidified, adhesiveness of the toner is reduced, and the recording sheet can be separated easily from the conveyance member. The solidification in this case is expected not to cause any degradation of the smoothness of the toner image surface.
Because the operation to assimilate the smooth surface of the belt to the toner image surface is performed in a state in which the toner image surface is softened, after the belt surface smoothness is transferred to the toner image surface, the cooling needs to be forcibly performed to solidify the toner, reduce the adhesiveness between the belt and the toner, and improve releasability of the sheet from the belt.
In the disclosed documents, a structure in which the cooling member such as a heat pipe or a heatsink in addition to sending air by a cooling fan is constantly in contact with the belt, which has passed through the heater section for smoothing the toner image surface, is used. However, such a structure has the following drawbacks.
First, because the cooling member is constantly in contact with the belt, the temperature of the belt which has passed through the heater section decreases, and, when reaching the heater section again, the belt mostly has a heat less than the temperature necessary to smooth the surface of the toner image as desired. Then, the temperature-raising time for the heater section increases and the more power consumption is required to raise the temperature of the heater section.
Second, when the cooling member remains in contact with the belt, the cooling member prevents the belt from moving freely and slippage occurs between the belt and the rollers around which the belt is wound.
For example, in a case in which a heating nip is formed using the pressure roller and the heat roller disposed in the heating section, if the belt is separated from the heat roller to prevent excessive heating of the pressure roller, the belt receives friction resistance from the cooling member due to the contact with the cooling member, resulting in slippage between the belt and the drive roller and a loss of coupled operation with the heat roller. As a result, heating and thermal capacity applying time to the recording sheet becomes unstable and a desired glossiness cannot be obtained.
In addition, resistance of the cooling member to the movement of the belt causes abrasion of the interior surface of the belt, and when the abrasion increases, a thickness of the belt changes and uniform contact between the belt and the recording sheet is prevented. As a result, it becomes difficult to obtain a smoothness of the toner image surface smoothed by the smoothness of the belt surface. Thus, the desired glossiness depending on the smoothness of the belt would not be obtained.